Artificial heart valve

ABSTRACT

A flexible leaflet heart valve (1), to replace natural aortic or pulmonary valves of the heart, includes a frame (3) and flexible leaflets (2) attached to the frame (3). Each flexible leaflet (2) forms part of a surface of revolution having its axis of revolution substantially orthogonal to the direction of blood flow through the valve (1). The valve (1) has improved opening characteristics under low flow conditions, and allows a large range of geometries for the same size valve.

BACKGROUND OF THE INVENTION

The present invention relates to artificial heart valves, and moreparticularly to flexible leaflet heart valves which are used to replacethe natural aortic or pulmonary valves of the heart.

Conventionally, ball or disk valves are used to replace natural mitralor tricuspid aortic or pulmonary valves of the heart. These artificialvalves comprise a rigid frame defining an aperture and a cage enclosinga ball or a disk. When blood flows in the desired direction, the ball ordisk lifts away from the frame allowing the blood to flow through theaperture. The ball or disk is restrained by the cage by struts or by apivot. When blood tries to flow in the reverse direction, the ball ordisk becomes seated over the aperture and prevents the flow of bloodthrough the valve. The disadvantage of these valves is that the ball ordisk remains in the blood stream when the blood flows in the desireddirection, and this causes a disturbance to blood flow.

More recently, flexible leaflet valves have been proposed which mirrornatural heart valves more closely. These valves have a generally rigidframe and flexible leaflets attached to this frame. The leaflets arearranged so that, in the closed position, each leaflet contacts itsneighbour thereby closing the valve and preventing the flow of blood. Inthe open position, the leaflets separate from each other, and radiallyopen out towards the inner walls of an artery in which the valve islocated. The leaflets are either made from chemically treated animaltissue or polyurethane material. The leaflets must be capable ofwithstanding a high back pressure across the valve when they are in theclosed position, yet must be capable of opening with the minimumpressure across the valve in the forward direction. This is necessary toensure that the valve continues to correctly operate even when the bloodflow is low, and to ensure that the valve opens quickly when blood flowsin the desired direction.

A wide range of geometries are used to describe natural aortic valveleaflets during diastole, but these geometries cannot be used for valvesmade from pericardial or synthetic materials due to the approximatelyisotropic properties of such materials compared to the highly anistropicmaterial of the natural valve. Consequently, different geometries haveto be used to form flexible leaflet heart valves made from pericardialor synthetic materials with isotropic mechanical properties.

Conventional flexible leaflet heart valves have three substantiallyidentical leaflets mounted onto the frame. The leaflets have a range ofdesigns, both in the geometry of the leaflet and the variations inthickness of the leaflets. Original flexible leaflet heart valvesincorporate leaflets which are spherical or conical when in the relaxedstate, that is when no pressure is acting on the leaflet. More recently,cylindrical and ellipsoidal leaflets have been proposed. These leafletgeometries are formed with an axis of revolution in a plane generallyparallel to the blood flow through the valve.

DISCLOSURE OF THE INVENTION

According to the present invention, a flexible leaflet heart valve forcontrolling the flow of blood comprises a substantially rigid frame anda plurality of substantially identical flexible leaflets mounted on theframe, characterised in that each flexible leaflet forms part of asurface of revolution having its axis of revolution lying in a planesubstantially orthogonal to the direction of blood flow through thevalve, and a shape defined by the equation:

    z.sup.2 +y.sup.2 =2R.sub.L (x-g)-α(x-g).sup.2

where:

g is the offset of the leaflet from the axis of the frame;

R_(L) is the radius of curvature of the leaflet at (g,0,0); and

α is the shape parameter and is greater than 0 and less than 1.

A flexible leaflet valve according to the present invention has improvedopening characteristics under low flow conditions. The shape of theleaflets is such that the radius of curvature of the leafletcontinuously increases in two directions away from the centre point ofthe free edge. By varying the radius of curvature, the leaflet shape maybe varied and still fit within the frame.

The value of α may be in the range of 0.2 to 0.8, but is preferably inthe range 0.4 to 0.6, and more preferably is about 0.5.

Preferably, x is in the range of 0 to R_(L), y is in the range between-R_(L) and +R_(L), and z lies in the range -1.8 R_(L) to +0.2 R_(L).

The valve preferably includes three leaflets, and in this case the valveclosure is preferably effected by the surface adjacent the free edge ofeach leaflet making sealing contact with the two neighbouring leaflets.The frame on which the leaflets are mounted is preferably circular incross-section, and has a size dependent upon the size of the aorta orpulmonary artery in which the valve is to be used.

Preferably, each leaflet is made from a polyurethane material, and has avariable thickness, preferably between 0.15 mm and 0.25 mm.

BRIEF DESCRIPTION OF THE DRAWINGS

An example of a flexible leaflet heart valve according to the presentinvention will be described with reference to the accompanying drawings,in which:

FIG. 1 shows an overall view of the valve;

FIG. 2 shows a plan of the valve; and,

FIG. 3 shows a cross section of one leaflet of the valve taken along theline A--A shown in FIG. 2.

DESCRIPTION OF PREFERRED EMBODIMENT

As shown in FIG. 1, a flexible leaflet heart valve 1 includes threeflexible leaflets 2 which are substantially identical to each other. Theleaflets 2 have a free edge 4. The leaflets 2 are mounted symmetricallyon a frame 3. The valve 1 is positioned in an artery with the axis ofthe frame 3 generally co-axial to the axis of the artery, and hence inthe same direction as the blood flow along the artery. The leaflets 2form part of a paraboloid having its axis of revolution lying in a planeorthogonal to the direction of blood flow.

Using cartesian geometry with the z direction being the direction ofblood flow, the y direction orthogonal to this and extending from thecentre of the free edge 4 of the leaflet 2, and the x direction beingorthogonal to both the y and z directions, then the shape of the leafletis represented by the equation:

    z.sup.2 +y.sup.2 =2R.sub.L (x-g)-α(x-g)

where:

g is the offset of the leaflet from the axis of the frame as shown inFIG. 3;

R_(L) is the radius of curvature of the leaflet at (g, 0, 0) as shown inFIG. 2; and

α is the shape parameter, and is greater than 0 and less than 1.

If α=1 the geometry of the leaflet 2 will be spherical. When α=0, thesurface will be parabolic. However, for 0>α>1, the leaflet shape has avariable radius of curvature having its axis of revolution in the x, yplane. This allows leaflets to be produced having a shape to give therequired properties which also fits within any given frame.

The valve radius, R_(F) as shown in FIG. 3, and R_(L) are both in therange of 5 mm to 20 mm, g is in the range of 0 to 3 mm and α is 0.5.

Leaflets of this shape open radially away from the centre of the frameout towards the wall of the artery with a very low pressure, typicallybelow 1 mm Hg. This is important as if the valve 1 fails to open at lowpressures, the blood will cease to circulate. The shape of the leaflets2 also ensures that they rapidly close when the blood tries to flow inthe reverse direction, therefore quickly preventing the blood fromflowing in this direction.

Various sizes of frame 3 may be used depending upon the size of theartery. Due to the leaflets 2, a frame radius R_(F) of about 13.5 mmproduces a valve having an effective orifice area of approximately 2.5cm². This typically allows approximately 4.5 liters per minute of bloodto flow through, at which rate, the valve 1 has a closing regurgitantvolume of less than 3 ml per stroke.

Although not shown, the valve may have only two or more than threeflexible leaflets 2.

The precise size and shape of the leaflets 2 depends upon the particularsize of vessel in which the valve 1 is to be used. In particular, theshape parameter α of the leaflets 2, may be varied to produce a set ofvalves 1 of substantially the same size but different shapes to suitmost applications. Alternatively, a set of heart valves 1 may beproduced all of which have the same shape, but have different sizes forparticular applications.

I claim:
 1. A flexible leaflet heart valve for controlling the flow ofblood in an artery comprising a substantially rigid frame (3), and aplurality of substantially identical flexible leaflets (2) mounted onthe frame (3), characterised in that each flexible leaflet (2) is formedfrom part of a surface of revolution having its axis of revolution lyingin a plane substantially orthogonal to the direction of blood flowthrough the valve (1), and a shape defined by the equation:

    z.sup.2 +y.sup.2 =2R.sub.L (x-g)-α(x-g).sup.2

where: g is the offset of the leaflet from the axis of the frame; R_(L)is the radius of curvature of the leaflet (2) at (g, 0, 0); and α is theshape parameter and is greater than 0 and less than
 1. 2. A flexibleleaflet heart valve according to claim 1, in which α is in the range 0.2to 0.8.
 3. A flexible leaflet heart valve according to claim 2, in whichα is within the range 0.4 to 0.6.
 4. A flexible leaflet heart valveaccording to claim 3, in which α is substantially 0.5.
 5. A flexibleleaflet heart valve according to claim 1, in which x is in the range 0to R_(L), y, when present, is in the range -R_(L) to +R_(L), and z is inthe range -1.8 R_(L) to +0.2 R_(L).
 6. A flexible leaflet heart valveaccording to claim 1, in which R_(F), which is a radius of the frame ofthe valve, and R_(L) are both between 5 mm and 20 mm, and g is in therange 0 to 3 mm.
 7. A flexible leaflet heart valve according to claim 1including three flexible leaflets (2).
 8. A flexible leaflet heart valveaccording to claim 1, in which the rigid frame (3) has a substantiallycircular cross section.
 9. A flexible leaflet heart valve according toclaim 1, in which the leaflets are made from polyurethane and have avariable thickness in the range 0.15 mm to 0.2 mm.